Ignition interlock system

ABSTRACT

An interlock circuit grounded at one side and having one wire connected to the ignition magneto of a gasoline engine. A second wire is connected to a switch on the transmission to actuate the circuit to be a short circuit across the magneto when the engine is not running, except when the transmission is in neutral. The circuit automatically disables itself after the engine starts.

EJ111106 Smiles Patent 1 1 Botker 1 51 Feb. 27, 1973 1 IGNITION INTERLOCK SYSTEM 3,498,282 3/1970 Dattilo ..123/198 1) 3,521,612 7/1970 Santi et al. .....l23/179 BG [75] Invent Kurt Bmke" Thummm 2,519,801 8/1950 Tognola ..l23/l48 [73] ASsignee: North American Philips Corpora- 2,934,054 4/1960 Quinlan ..l23/179 tion, New York, NY. Prima Examiner-Laurence M. Goodrid e 22 F] d: A 7 g 1 16 19 1 Attorney-Donald P. Gillette [21] Appl. No.: 173,531

[57] ABSTRACT 123/148 123/179 1 A11 interlock circuit grounded at one side and having 123/198 D one wire connected to the ignition magneto of a Cl. gasoline engine A second wi e is connected to a [58] Field of Search.123/148, 198 D, 179 BG, 179 K switch on the transmission to actuate the circuit to be 56 a short circuit across the magneto when the engine is Reterences and not running, except when the transmission is in UNITED STATES PATENTS neutrah The circuit automatically disables itself after the engine starts. 3,675,036 7/1972 Davies ..123/198 D 3,601,103 8/1971 Swiden ..l23/198 D 11 Claims, 3 Drawing Figures w, /4 T /3 f]; II jl 1:1 Z6 Z7 35 A24 1' p 1?. L 1

PATENTEDFEBZYIQB 3,718,128

ll 24 r $5 [8 2/ 19 L INVENTOR, Kurt BOJK k @r IGNITION INTERLOCK SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to interlock controls for engines having magneto ignition and particularly to a solid-state interlock circuit requiring only one connection to the magneto and one connection to a transmission switch.

2. The Prior Art Interlock systems for magneto ignition engines have heretofore used relays having normally closed contacts in series with normally closed switch contacts on a transmission switch to short-circuit the primary of the magneto unless the transmission was in neutral. In that case, the series circuit would be opened and the engine could becranked so as to produce magneto impulses that would provide the necessary operating spark. The coil of the relay was connected to the primary of the magneto to receive relatively low voltage pulses from it. These pulses were rectified to provide an operating current for the coil to open the normally closed relay contacts as long as pulses were supplied. This opening of the normally closed relay contacts would also open the series circuit, thereby making it possible to shift the transmission into any other gear without closing the series circuit and short-circuiting the magneto.

One of the disadvantages of the foregoing circuit is that it required two connections to the switch mounted on the transmission, and if either of these connections was broken, either deliberately or by accident, the effect on the remainder of the circuit would be the same as if the transmission had been placed in neutral. Thus by breaking the connections, the interlock would be defeated and the engine could be cranked whether it was in neutral on in some forward speed or in reverse.

It is an object of the present invention to provide an interlock circuit having simple connections to the transmission of a magneto-ignition engine and capable of preventing the engine from being starting except when the transmission is in neutral.

Another object is to provide such an interlock circuit in a form directly connectable to the engine and having only a single wire to be connected to a transmission switch, which must be closed to permit starting of the engine only in neutral.

Further objects will become apparent from the following specification and the drawings.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the invention, a solid-state circuit is provided in a form that can be connected to ground and which has one connection leading to the ignition magneto and a second wire leading to a switch preferably close to and actuated by the transmission control so that the switch will be closed only when the transmission is in neutral. The circuit includes means to rectify the impulses from the primary of the magneto and to use those pulses to produce a voltage that will turn off a field-effect transistor (FET). The field-effect transistor in turn controls the operation of a transistor connected to the gate of a silicon-controlled rectifier (SCR) so that the SCR will be non-conductive if the FET is conductive. A switch actuated by the transmission control is closed when the transmission is in neutral and provides an additional means for keeping the SCR from conducting as long as the switch is closed. The SCR is connected directly across the magneto primary to short-circuit the magneto when'the SCR is conductive. Once the engine is running, the i switch can be opened by shifting the transmission into any gear and the SCR will continue to be kept in the non-conductive state by the operation of the FET. However, if an attempt is made to start the engine with the transmission not in neutral, the SCR will be turned on at the beginning of each magneto impulse and will short-circuit the primary of the magneto to prevent the build-up of sufficient voltage for ignition of the engine.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the ignition control system of the present invention.

FIG. 2 is a schematic diagram of the ignition interlock circuit for use in the system of FIG. 1.

FIG. 3 is a schematic diagram of a modified interlock circuit for use in the system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION The engine system in FIG. 1 comprises a gasoline engine 11 having a magneto ignition system with the magneto indicated by reference numeral 12. The engine is typically of the type used in small tractors, lawnmowers, and other devices using air-cooled engines, although it may be used with engines intended for more complex machinery, and size is not a limiting factor in the utility of the invention. The engine 11 has a drive shaft 13 connected to a transmission 14 having a shift lever 16 that operates in a gate 17. The gate illustrated in FIG. 1 is a typical I-I pattern, but it is not necessary that the gate be this complex. All that is required is that the transmission lever 16 be capable of being moved to a predetermined position defined as the neutral position so that the engine 11 can be started free of any load imposed by the mechanism it is intended to drive. For an example, in the case of such things as tractors, the machine must not be started in gear, which might cause it to run away from the operator. The engine has an ignition shorting switch 18 connected to the primary of the magneto 12 to short-circuit the magneto in order to stop the production of further ignition impulses and thereby cause the engine to stop.

The interlock used with the engine 11 includes a circuit 19 connected by means of a single connector 21 to the magneto primary l2 and connected by another single wire 22 to a normally open transmission switch 23. The switch 23 is physically attached to the housing of the transmission 14 and is connected to the control lever 16 by any convenient means, such as a bar 24, that is depressed when the lever 16 is in neutral. This closes the switch 23 only when the lever is in that position but allows it to open when the lever is in any other position. It may be desirable to provide additional interlock switches to be actuated by other parts of the mechanism, and three such switches 26-28 are shown connected in series with the switch 23.

The operation of the system in FIG. 1 is such that the magneto 12 can supply ignition pulses to the engine 11 only when the switch 18 is open and the circuit 19 is also the equivalent of an open circuit across the magneto. The engine 11 can be stopped by the switch 18, which is usually a manually controlled switch. The engine can be prevented from ever starting if the circuit 19 forms the equivalent of a short circuit between the wire 21 and ground. Prior to the time the engine 11 is started, the condition of the circuit 19 is such that it will become in effect a short circuit between the wire 21 and ground in response to the beginning of each ignition impulse unless the circuit 19 is held in an open circuit condition by keeping the switch 23, and each interlock switch 26-28 connected in series therewith, closed. After the engine 1 1 has been started up, the circuit 19 is no longer dependent on the switch 23 and the latter can be opened.

In order to minimize the connections from the engine 11, the circuit 19 may be enclosed within a housing directly attached to the engine 1 1. In that case, the only wires leading from the engine would be the wire to the switch 18 and the wire 22 to the switch 23. The wire to the switch 18 is standard on such engines and thus the only additional wire necessitated by the interlock system in FIG. 1 would be the wire 22. Because the switches 23 and 26-28 would have to be closed for the circuit 19 to be placed in condition to allow the engine 1 1 to start, anything that interrupted the circuit, as for example severing the wire 22, would make starting impossible. Thus, the interlock system cannot be defeated simply by disconnecting the wire 22.

FIG. 2 shows the circuit 19 of FIG. 1 arranged to be actuated by voltage impulses from the primary of the magneto 12 that provides ignition voltage for the engine 11 in FIG. 1. These pulses include both positivegoing and negative-going pulse components, and in this embodiment it is the negative-going pulses that are used in the circuit in FIG. 2. These pulses are applied to the circuit by way of the conductor 21 so that the circuit is, in effect, connected directly in parallel with the primary of the magneto. The input signal that actuates the circuit 19 and the output controlling efiect on the magneto both exist between the same two terminals, i.e., the conductor 21 and ground. This output effect is simply that the circuit 19 can act either like a short circuit or an open circuit between the conductor 21 and ground.

At the left hand side of the circuit in FIG. 2 is a rectifying circuit comprising a diode 31 connected in series with a capacitor 32 and a resistor 33 between ground and the conductor 21. Another resistor 34 is connected in parallel with the part of the series circuit comprising the capacitor 32 and the resistor 33. A diode 36 and a capacitor 37 are connected in series with each other and in parallel with the resistor 33. A high impedance capacitor discharge resistor 38 is connected directly in parallel with the capacitor 37, and this parallel circuit is connected between the gate and source electrodes of a PET 39 of the N-channel depletion type. The source and drain electrodes of the FET 39 are connected in parallel with a resistor 41, which is one of the elements of a voltage divider comprising the resistor 41 and another resistor 42. This voltage divider is connected in series with a diode 43 between ground and the conductor 21. A diode 44 is connected to the junction between the resistors 41 and 42 and in series with a resistor 46 to the base of a transistor 47. The transmission switch 23 and the connector 22 are connected in series with a resister 48 and a diode 49 between ground and the base of the transistor 47. One end of another resistor 51 is connected to the junction between the resistor 48 and the diode 49. The other end of the resistor 51 is connected to the conductor 21 The emitter of the transistor 47 is connected to the conductor 21 and the collector is connected to the midpoint of a voltage divider comprising a resistor 52 and another resistor 53. These resistors are connected in series with the diode 43 and in parallel with the resistors 41 and 42, and the midpoint of this voltage divider is directly connected to the gate electrode of an SCR 54. The anode and cathode circuit of the SCR 54 is connected directly between the conductor 21 and ground.

Operation of the circuit in FIG. 2 will first be described with the switch 23 closed as it must be to allow the engine 11 in FIG. 1 to start. The circuit is arranged to operate in response to one of the aforesaid negative-going impulses produced each time the magneto 12 fires. When this happens, the diode 31 conducts and the capacitor 32 is charged by the negative voltage pulse so that the upper plate of the capacitor, as shown in FIG. 2, becomes positive with respect to the lower plate. When the negative pulse begins to decrease in magnitude, the diode 31 becomes non-conductive and the capacitor 32 begins to discharge through the series circuit comprising the resistors 33 and 34. As a result of this discharge current, the upper terminal of the resistor 33 becomes negative with respect to the lower terminal. Furthermore, due to the ratio of resistances of the voltage divider, the voltage across the resistor 33 is a fraction of the voltage across the capacitor. A typical fraction would be about onetenth of the voltage across the capacitor, and, in fact, typical values for the latter voltage are between 50 and 200 volts. Thus, the voltage across the resistor 33 is typically about 5 to 20 volts.

The voltage across the resistor 33 is of the correct polarity to cause the diode 36 to conduct and to charge the capacitor 37 to the same voltage as is present across the resistor 33. This capacitor may not charge fully with the first pulse but may require several pulses, which is the same thing as saying it may require several revolutions of the crank shaft of the motor 1 l in FIG. 1 The voltage across the capacitor 37 is applied directly across the gate and source electrodes of the FET 39. Initially, the PET is conductive and virtually short-circuits the resistor 41; but as the capacitor 37 charges, the voltage between the gate and source electrodes reaches the level at which the FET is no longer conductive, and thereafter the FET 39 and all elements of the circuit to the left of it are of no further effect on the operation of the remainder of the circuit 19 shown in FIG. 2 as being to the right of the FET.

As the gasoline engine 1 1 continues to tum-over and the magneto 12 continues to produce voltage impulses, these impulses pass through the diode 43 and are applied across the first voltage divider comprising the resistors 41 and 42. Since the resistor 41 is no longer short-circuited, a fraction of this impulse voltage appears across the resistor 41 and is transmitted by the diode 44 and the current-limiting resistor 46 to the base of the transistor 47. In response to each of these pulses, the transistor 47 becomes conductive so that the voltage on its collector drops to approximately the same voltage as the emitter. In effect, this short-circuits the resistor 53, which is connected between the gate of the SCR 5d and the cathode, and makes it impossible for the SCR to conduct.

The same voltage impulses are applied through the diode 43 and across the voltage divider comprising the resistors 52 and 53 so the fraction of this voltage across the resistor 53 would normally be applied between the gate and cathode of the SCR 54. However, the transistor 47 becomes conductive virtually instantaneously and before the SCR 54 is able to respond, and, therefore, the resistor 53 is short-circuited by the transistor 47 before the voltage across it can build up to a sufficient level to cause the SCR 54 to conduct.

As long as the switch 23 is closed, voltage impulses from the magneto 12 will also be applied across the voltage divider comprising the resistors 48 and 51. Depending on the relative division ratio of this voltage divider as compared with the voltage divider comprising the resistors 41 and 42, voltage impulses may be transmitted by way of the diode 49 to the base of the transistor 47 causing the transistor to conduct. Thus, no matter whether the voltage impulses are transmitted through the diode 43 to the base of the transistor $7 or through the diode 49 to the base of the transistor 47, the transistor 47 will conduct at the beginning of each voltage impulse from the magneto 12. Even after the switch 23 is opened by shifting the transmission control 16 in FIG. 1 to some position other than neutral, the transistor 47 will continue to conduct at the beginning of each magneto impulse and will continue to prevent the SCR 54 from becoming conductive and short-circuiting the magneto 12.

Next, the operation of the circuit will be described under the condition that starting is attempted with the transmission control 16 of FIG. 1 in some position other than neutral. In that case, the switch 23 will be open. It may be noted that the same result would be obtained if any of the switches 264% in FIG. I were open even though the transmission was in neutral. With the switch 23 open, the transistor 47 receives no impulses through the diode 49 Initially, as in the previous instance, the FET 39 is conductive so that the resistor 41 is effectively short-circuited, which prevents voltage impulses from being produced across this resistor and transmitted through the diode 44 to the transistor 47. As a result, initially no impulses are applied to the base of the transistor 47, and this transistor is not conductive. However, voltage impulses passing through the diode 43 are applied across the voltage divider comprising the resistors 52 and 3, and that fraction of these voltage impulses across the resistor 53 is applied to the gate of the SCR 54 and causes the SCR 54 to conduct. Although this conduction does not start at the instant the voltage impulses start, it does begin soon enough thereafter to prevent any substantial build-up of voltage by the magneto l2 and, therefore, prevents ignition impulses from being supplied to the engine 111 in FIG. 1. As a result, no matter how long the engine 11 is cranked, it will not start with the switch 23 or any of the other switches 26-28 open.

Some engines are designed to operate with positivegoing magneto voltage impulses, which are the converse of the negative-going impulses referred to heretofore. This requires a modification in the interlock circuit, although the system circuit diagram remains the same as in FIG. 1. FIG. 3 shows the modified circuit suitable for use with an engine having positive-going magneto impulses. In this case, the interlock circuit is identified by reference numeral 119 to distinguish it from the circuit 19 in FIG. 2, and the magneto is identified by reference numeral 112 for the same reason. The magneto 112 has a so-called hot or voltage supplying terminal and a ground terminal. It will be noted that, contrary to the external connections of the circuit in FIG. 2, the diodes 31 and 43 in FIG. 3 are connected through the conductor 21 to the magneto 112, and the source of the FET 39, the emitter of the transistor 47, and the cathode of the SCR 54 are connected to ground, which is usually the frame of the machine. Thus, the pulses applied to the circuit in FIG. 3 connected in this way are of the same relative polarity as those applied to the reversely connected circuit in FIG. 2.

It will be noted in FIG. 3 that the switch 23 is directly connected between the gate and cathode of the SCR 54 and that there are no diodes equivalent to the diodes 44- and d9 in FIG. 2 nor is there a voltage divider comparable to the voltage divider comprising the resistors 48 and 51 in FIG. 2.

The operation of the circuit in FIG. 3 will first be described as if the switch 23 were closed so as to place the circuit in proper condition for allowing the engine ill in FIG. 1 to start. With the switch 23 closed, the gate of the SCR 54 is short-circuited to the cathode and the SCR cannot conduct and, therefore, cannot short-circuit the magneto 112. As long as this switch remains closed, the engine can be started and will continue to run.

As it runs, it supplies voltage impulses of the correct polarity to be transmitted through the diode 31 and to charge the capacitor 32. Between these voltage impulsea the capacitor 32 discharges by way of the resistors 33 and 34 so that a fraction of the voltage across the capacitor 32 is transmitted through the diode 36 to build up a voltage across the capacitor 37. As in the circuit in FIG. 2, the build-up of voltage across the capacitor 37 produces a staircase-type of waveform. Initially, the FET 39 is conductive and effectively short-circuits the resistor All, but after several revolutions of the engine, the staircase waveform across the capacitor 37 reaches a level that causes the FET to become nonconductive. From then on, magneto impulses transmitted through the diode 33 produce positive-going impulses across the resistor 41 and cause the transistor 47 to become conductive at the beginning of each of these positive-going impulses. Each time this happens, the transistor 47 effectively short-circuits the gate of the SCR 54 to the cathode. This short circuit is in parallel with the short circuit provided by the switch 23 as long as the switch remains closed; but the short circuit provided by the transistor continues to be operative even after the gear shift has been moved to a position other than neutral and the switch 23 has opened. I

Finally, the operation of the circuit in FIG. 3. will be described with the switch 23 initially open as it would be if the transmission control lever 16 of FIG. 1 were not in the correct position for starting the engine.

Initially, the FET 39 is conductive, which effectively short-circuits the resistor M and prevents the transistor 47 from receiving any impulses that would cause it to become conductive. As a result, the gate of the SCR 54 is not short-circuited to the cathode and the same voltage impulses from the magneto 112 are supplied through the diode 43 and across the voltage divider comprising the resistors 52 and 53. The fraction of this voltage that appears across the resistor 53 is sufficient to cause the SCR S4 to become conductive and thus to short out the magneto 112 and prevent full build-up of the voltage necessary for ignition of the engine 11 in FIG. 1. This prevents the engine from starting. It also prevents the magneto from producing pulses of the necessary magnitude to charge the capacitor 37 to turn off the FET 39.

Because of the reversal in polarity of the voltage impulses from the magneto 112 in FIG. 3 as compared to the voltage impulses from the magneto 12 in PEG. 2, the sensing switch 23 that senses whether the transmission 14 in FIG. 1 or other power take-up device is in proper condition for starting can be directly connected to the gate and cathode of the SCR 54, thus eliminating the more complex circuit by way of which the switch 23 is connected in the circuit 19 in FIG. 2. As a result, there is only one voltage divider which consists of the resistors 41 and 42 connected to the input circuit of the transistor 47. Without the second voltage divider consisting of the resistors 48 and 51 in FIG. 2, there is no need to have the diode 44 that effectively separates these resistors from the voltage divider consisting of the resistors 41 and 42.

What is claimed is:

l. A circuit to control the starting of a magneto ignition engine having power take-off means movable between at least one safe condition and one other condition and in which the magneto has a voltage supplying terminal and ground, said circuit comprising:

A. A silicon-controlled rectifier comprising:

1. an output circuit connected between said terminal and ground to short-circuit said magneto and prevent it from producing ignition pulses when said rectifier is conductive, and

2. an input circuit;

B. A transistor comprising:

1. an output circuit connected across the gate and cathode terminals of said silicon-controlled rectifier to prevent said rectifier from becoming conductive when said transistor is conductive, and

2. an input circuit;

C. Means to supply voltage impulses from said magneto to said rectifier to tend to cause said rectifier to become conductive in response to each of said impulses;

D. Means to supply voltage impulses from said magneto to said transistor to cause said transistor to become conductive upon the occurrence of each of said impulses prior to said rectifiers becoming conductive;

E. Means to prevent said last-named impulses from making said transistor conductive until the engine is running; and

F. A switch having one side connected to ground and being mechanically controlled by the power takeoff mechanism to be closed only when the power take-off mechanism is in said safe condition to allow the engine to start, the other terminal of said switch being connected to said transistor to effect a short-circuit condition across said input circuit of said silicon-controlled rectifier at least during the occurrence of each of the voltage impulses from said magneto.

2. The circuit of claim 1 in which said switch is connected inparallel with the output circuit of said transistor.

3. The circuit of claim 1 in which said switch is connected between ground and said input circuit of said transistor.

4. The circuit of claim 3 comprising, in addition:

A. A first .voltage divider connected in series with said switch and said terminal of said magneto; and

B. A connection between one part of said voltage divider and said input circuit of said transistor.

5. The circuit of claim 4 comprising, in addition, a first diode, connected between said voltage divider and said input circuit of said transistor to permit voltage impulses of only one polarity to be applied to said transistor.

6. The circuit of claim 5 comprising, in addition:

A. A second diode and a second voltage divider connected in series between said magneto terminal and ground; and

B. A connection between one section of said second voltage divider and said input circuit of said transistor to provide a second circuit for supplying pulses from said magneto to said transistor.

7. The circuit of claim 6 comprising, in addition, a second diode connected in series between said section of said second voltage divider and said input circuit of said transistor and polarized to prevent undesired coupling between said first and second voltage dividers.

8. The circuit of claim 6 comprising, in addition:

A. A semiconductor device connected in parallel with said portion of said second voltage divider; and

B. Means responsive to repetitive voltage pulses from said magneto to render said semiconductor device non-conductive.

9. The circuit of claim I in which said means to prevent said impulses from making said transistor conductive until the engine is running comprise:

A. A semiconductor device comprising:

1. a normally conductive output circuit connected in parallel with said input circuit of said transistor, and

2. an input circuit;

B. A capacitor connected across said input circuit of said semiconductor device; and

C. A charging circuit connected to said capacitor to supply repetitive incremental charges thereto at each firing of said magneto to charge said capacitor to a voltage level to render said semiconductor device non-conductive.

10. The circuit of claim 9 in which said charging circuit comprises:

A. A second capacitor;

B. A first diode connecting said second capacitor to said magneto; and

C. A second diode connecting said second capacitor to said first-named capacitor to transfer charge from said second capacitor to said first-named diode; and B. A second resistor connected in series with said second capacitor and said first resistor in a second closed loop. 

1. A circuit to control the starting of a magneto ignition engine having power take-off means movable between at least one safe condition and one other condition and in which the magneto has a voltage supplying terminal and ground, said circuit comprising: A. A silicon-controlled rectifier comprising:
 1. an output circuit connected between said terminal and ground to short-circuit said magneto and prevent it from producing ignition pulses when said rectifier is conductive, and
 2. an input circuit; B. A transistor comprising:
 1. an output circuit connected across the gate and cathode terminals of said silicon-controlled rectifier to prevent said rectifier from becoming conductive when said transistor is conductive, and
 2. an input circuit; C. Means to supply voltage impulses from said magneto to said rectifier to tend to cause said rectifier to become condUctive in response to each of said impulses; D. Means to supply voltage impulses from said magneto to said transistor to cause said transistor to become conductive upon the occurrence of each of said impulses prior to said rectifier''s becoming conductive; E. Means to prevent said last-named impulses from making said transistor conductive until the engine is running; and F. A switch having one side connected to ground and being mechanically controlled by the power take-off mechanism to be closed only when the power take-off mechanism is in said safe condition to allow the engine to start, the other terminal of said switch being connected to said transistor to effect a short-circuit condition across said input circuit of said silicon-controlled rectifier at least during the occurrence of each of the voltage impulses from said magneto.
 2. an input circuit; B. A transistor comprising:
 2. an input circuit; C. Means to supply voltage impulses from said magneto to said rectifier to tend to cause said rectifier to become condUctive in response to each of said impulses; D. Means to supply voltage impulses from said magneto to said transistor to cause said transistor to become conductive upon the occurrence of each of said impulses prior to said rectifier''s becoming conductive; E. Means to prevent said last-named impulses from making said transistor conductive until the engine is running; and F. A switch having one side connected to ground and being mechanically controlled by the power take-off mechanism to be closed only when the power take-off mechanism is in said safe condition to allow the engine to start, the other terminal of said switch being connected to said transistor to effect a short-circuit condition across said input circuit of said silicon-controlled rectifier at least during the occurrence of each of the voltage impulses from said magneto.
 2. The circuit of claim 1 in which said switch is connected in parallel with the output circuit of said transistor.
 2. an input circuit; B. A capacitor connected across said input circuit of said semiconductor device; and C. A charging circuit connected to said capacitor to supply repetitive incremental charges thereto at each firing of said magneto to charge said capacitor to a voltage level to render said semiconductor device non-conductive.
 3. The circuit of claim 1 in which said switch is connected between ground and said input circuit of said transistor.
 4. The circuit of claim 3 comprising, in addition: A. A first voltage divider connected in series with said switch and said terminal of said magneto; and B. A connection between one part of said voltage divider and said input circuit of said transistor.
 5. The circuit of claim 4 comprising, in addition, a first diode connected between said voltage divider and said input circuit of said transistor to permit voltage impulses of only one polarity to be applied to said transistor.
 6. The circuit of claim 5 comprising, in addition: A. A second diode and a second voltage divider connected in series between said magneto terminal and ground; and B. A connection between one section of said second voltage divider and said input circuit of said transistor to provide a second circuit for supplying pulses from said magneto to said transistor.
 7. The circuit of claim 6 comprising, in addition, a second diode connected in series between said section of said second voltage divider and said input circuit of said transistor and polarized to prevent undesired coupling between said first and second voltage dividers.
 8. The circuit of claim 6 comprising, in addition: A. A semiconductor device connected in parallel with said portion of said second voltage divider; and B. Means responsive to repetitive voltage pulses from said magneto to render said semiconductor device non-conductive.
 9. The circuit of claim 1 in which said means to prevent said impulses from making said transistor conductive until the engine is running comprise: A. A semiconductor device comprising:
 10. The circuit of claim 9 in which said charging circuit comprises: A. A second capacitor; B. A first diode connecting said second capacitor to said magneto; and C. A second diode connecting said second capacitor to said first-named capacitor to transfer charge from said second capacitor to said first-named capacitor upon the occurrence of each magneto pulse of the proper polarity.
 11. The circuit of claim 10 comprising, in addition: A. A first resistor connected in series with said second capacitor and in a closed loop series circuit with said first-named capacitor and said second diode; and B. A second resistor connected in series with said second capacitor and said first resistor in a second closed loop. 